This is about including a fix that was included in 3.11 and master only as part of a deprecation, in stables. The same bug exists in stables too.
The web service method "get_discussion_posts" will not properly include deleted posts as the timecreated value on the post is set to null (see https://github.com/snake/moodle/blob/883ded96ae59bd3b9497873035bd7fbc18e9c760/mod/forum/classes/local/exporters/post.php#L412)
You can find the commit which includes the fix here:
In addition to pulling the fix in, we'll want to include the test method included there too. Right now, this isn't a case that's covered in stables.